It's been a long road for Van Gaal and Jose Mourinho.
They started out alongside one another at Barcelona, then became rivals. The Chelsea boss was, at a point in his early career, Van Gaal's assistant in Spain, with the two helping each other adapt and improve, Mourinho in particular learning a lot from his mentor.
Now, they will be going head-to-head in the Premier league.
You won't see either man sharing a training session these days, although they will no doubt enjoy their post-match glasses of wine. They are still friends - but also great adversaries. And both have very big egos.
Ahead of Van Gaal's first job in England, the exchange of mind games has already begun between the two.
When the Dutchman got the job, he claimed that 51-year-old Mourinho told him that he was jealous. That was undoubtedly a joke shared in private between the two that Van Gaal knew he could manipulate for his benefit in public. Mourinho, of course, is no stranger to doing the same.
In the past, meanwhile, the current Netherlands boss has claimed that the Portuguese is the best coach of his generation - but only because he himself is 10 years older.
The two have already met as managers, notably in the 2010 Champions League final, where the 'pupil' defeated the 'master' as Inter Milan triumphed 2-0 against Bayern Munich in Mourinho's final game in charge of the Italian side.
The 'Special One' will relish Van Gaal's arrival.
Not since Sir Alex Ferguson has the Portuguese had a truly great rival to battle both on the field of play and in the media room - 'great' by his accounts, anyway.
Yes, there is Arsene Wenger of Arsenal - but Mourinho does not seem to hold him in the same regard since the Gunners' days of realistically challenging for the Premier League title ended. Wenger is a 'specialist in failure' after all, according to the 51-year-old.
There are Brendan Rodgers of Liverpool and Manuel Pellegrini of Manchester City, both of whom finished above Mourinho in the Premier League last season. But something seems to be missing from the respective rivalries.
That key ingredient is ego, something Mourinho has in abundance - and so too Van Gaal.
Throughout last season, the Chelsea boss kept reiterating his view that the Blues were not ready to challenge on all fronts until next term. For him, though, simply challenging and winning would be something he'd consider too easy.
Mourinho needs a personal angle: to be able to say 'yes, I showed him.'
With Van Gaal, he has exactly that - and it will drive him to new heights as a manager.
That's not to disrespect anyone else battling to topple Mourinho's Chelsea. But it's the way the 'Special One' operates. By that reckoning, Manchester United's new manager is the perfect catalyst for an all-out assault from the Blues boss.